The invention relates to a method for the determination of the 3D coordinates of an object and to an apparatus for the carrying out of such a method.
Methods and apparatus for the three-dimensional digitization of objects are already known. In this method, a pattern is projected onto the object by a projector. The pattern on the object is detected using one or more cameras. The projector and the camera or cameras can be integrated in a so-called 3D sensor from a construction aspect.
In particular the three-dimensional digitizing of objects by means of white light stripe projection is already known. In this process, a stripe image is projected onto the object to be digitized by a projector. The stripe image on the object is then detected by one or more cameras. The 3D coordinate of the object surface can be determined from this for the object region illuminated by the projector and detected by the camera or cameras.
To achieve good measurement results with respect to the three-dimensional measurement precision and to the noise of the measured data, the process requires a surface which reflects as ideally diffusely as possible (a so-called Lambert surface). In practice, however, the object whose 3D coordinates should be determined often have a shiny surface which does not reflect the incident light perfectly diffused, but rather more or less greatly directed. This is in particular the case with ground or polished metal surfaces or with shiny plastic surfaces.
To solve the problems associated with this, it is possible to apply a white, diffusely reflecting layer onto shiny surfaces to enable a determination of the 3D coordinates with a quality which is as high as possible. The layer can be applied using a spray. The coating of the surface of the object before the determination of the 3D coordinates, however, brings along some different disadvantages: An additional time and material effort arises and thus costs for the application and optionally the later removal of the layer from the object. The thickness of the applied layer can falsify the 3D coordinates of the object surface to be determined. No additional layer can be applied to sensitive materials since the object can hereby be damaged or destroyed. The application of the layer usually takes place manually and is therefore not practical in a measurement system in which a large number of objects should be measured automatically.
On the determination of the 3D coordinates of an object having a shiny surface, measurement errors can arise due to the pattern reflected in a directed manner at the object when the reflected light is again incident on another point on the object and is superimposed on the light projected directly onto the object at this point. This is in particular the case at inner edges of the object where the light reflected by a flank of the inner edge is partly incident onto the oppositely disposed flank. To illustrate this problem
The apparatus shown in FIG. 1 includes a projector 1 for the projection of a pattern, in particular of a stripe pattern, onto an object 3 and a camera 3 for the taking of the pattern reflected by the object 2. The surface of the object 2 has an inner edge which includes a horizontal flank 4 and a vertical flank 5 which impact at an inner corner point 11. The light from the projector 1 projected onto the horizontal flank 4 is partly reflected diffusely from the at least partly shiny surface of the horizontal flank 4 and is mirrored to another part at the vertical flank 5. The proportion of light reflected at the vertical flank 5 is partly reflected diffusely there and partly mirrored again. In particular the portion diffusely reflected at the vertical flank 5 is overlaid with that portion of the light from the projector 1 which is directly incident onto the vertical flank 5 and is diffusely reflected there. Both light portions arrive at the camera 3 and falsify the measurement.
For example, both the light ray 5 directly incident from the projector and the mirrored ray 8 of the light ray 9 directed to the horizontal flank 4 reach the point 6 of the surface of the vertical flank 5. On the image detection with the camera 3, the intensity of the light projected directly onto the object surface is detected for the respective object point and the intensity of the light of another object point (in the selected example, of the object point 10 on the horizontal flank 4 onto which the light ray 9 is incident) mirrored at this point by the shiny surface is detected.
The quality of the 3D coordinate data which should be determined is the poorer, the larger the proportion of the light not projected directly by the projector, but rather reflected indirectly at the object. This is shown schematically in FIG. 2 for the example of the inner edge of FIG. 1. The noise of the measured data is symbolized there by the amplitude of the vibration. The noise of the measured data is the greater, the smaller the spacing of the respective object point from the inner corner point 11 of the inner edge. FIG. 2 shows the measurement error in an exaggerated representation, with the measured data being shown by the solid line and the desired data by the dashed line.
A method is known from the older priority, not pre-published German patent application 10 2006 048 234 for the determination of the 3D coordinates of an object in which a stripe pattern is projected onto the object and the pattern reflected by the object is taken and evaluated.